When two fluorescent or molecular entities are joined together to form one molecule, they may, in some cases function as “energy transfer cassettes”. Molecules that act as energy transfer cassettes are characterized by transfer of all or part of the energy absorbed by one of the otherwise fluorescent parts to the other, which then fluoresces with enhanced brightness. The molecular fragment that absorbs energy then donates it to the other part may be referred to as the donor, while the molecular fragment that collects energy from the donor and emits with enhanced fluorescence is typically called the acceptor.
There are various mechanisms by which a donor entity may transfer energy to an acceptor entity in the same molecule. Most frequently, the energy is transferred almost exclusively through space via a mechanism often referred to as Förster energy transfer. Energy transfer cassettes based on through-space energy transfer are extremely common.
Through-bond energy-transfer (hereafter, TBET) cassettes are rarer than through-space energy-transfer cassettes (TSET). Nevertheless, some examples have been described. The most notable ones include models for photosynthetic systems, various studies on polymeric or oligomeric systems for new materials, and other model studies featuring boron dipyrromethane dyes.